What is the role of a Keystone species in an ecosystem?

A keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether.

In other words, Keystone species are those which have an extremely high impact on a particular ecosystem relative to its population. Keystone species are also critical for the overall structure and function of an ecosystem, and influence which other types of plants and animals make up that ecosystem.